In a stringed instrument, the vibration of the strings consists of transverse deflections (waves) that propagate longitudinally (i.e., along the length of the strings) in both directions. The motion of the strings in the surrounding air converts their elastic and kinetic energy into acoustic radiation and heat. Thus, the transverse waves are attenuated as they propagate along the strings.
At the ends of the strings, some acoustic power is transmitted into the supporting structure due to its slight elasticity. However, an acoustic impedance mismatch between a string and the structure generally causes a large fraction of the power in the incident waves to be reflected from each anchor point as waves travelling in the opposite direction along the string.
The strings are themselves inefficient acoustic radiators, but they do produce some air-borne sound directly. Although most of this sound radiates away from the instrument, some radiates onto its surface. A severe mismatch of the acoustic impedance of the solid surface and that of air causes most of the incident acoustic power to be reflected from the surface and back into the air. Therefore, only a very small amount of acoustic power is transmitted to the structure in this way.
In the structure, acoustic power is dissipated by radiation from the surface into the surrounding air and by internal damping (friction). A small amount of acoustic power is transmitted from the structure back into the strings through the anchor points. Reabsorption of airborne sound by the strings is negligible.
The flow of acoustic energy in a stringed instrument is shown schematically in FIG. 7. The circles labeled 120, 122, and 124 represent the strings, supporting structure, and air, respectively. The heavy (wide) and light (narrow) lines represent the primary and secondary acoustic power transmission paths, respectively. The broken (dashed) circle 125 and lines represent an optional electronic pickup (vibration transducer) and its primary and secondary acoustic inputs, respectively. It is assumed that the pickup is attached to the structure (as in conventional electric guitars) and is thus primarily sensitive to structure-borne sound. (Electromagnetic pickups sense string motion rather than structural vibration.)
In the structure, acoustic (elastic) waves can propagate along many different paths. The acoustic attenuation depends on the medium, path, and frequency. Hence, the materials and geometry of the structure influence the acoustic attenuation as a function of frequency which in turn determines the "tonal quality" or "tonality" of the instrument. (Tonal characteristics that musicians consider desirable depend, to a certain extent, on the style of music.) Multiple acoustic paths can also cause destructive interference (phase cancellation) of desirable frequencies. This effect is referred to as "multipath distortion".
It is thus apparent that acoustic coupling between the strings and the supporting structure and within the structure itself affects the quality of an "acoustic" (unamplified) instrument. In the case of an "electric" (amplified) instrument, its importance can be paramount. An acoustic coupling consideration of particular importance pertains to vibrato.
Vibrato is a slightly tremulous effect imparted to an instrumental tone for added warmth and expressiveness, consisting of slight and rapid variations in the pitch of the tone being produced. Stringed instruments, such as guitars, violins, violas, cellos, double basses, banjos, mandolins, together with a few other instruments such as trombones, are unique in allowing the musician to produce any of a continuum of musical pitches by making slight variations in the position of fingers or in the configuration of the instrument. Among stringed instruments, this has led to the development and use of techniques to produce vibrato sounds by varying the position of the fingers along the strings.
Another way to produce vibrato sounds is by using a vibrato assembly that varies the tension of the strings while the fingers remain stationary. A conventional vibrato assembly (often called a tremolo tailpiece even though in stringed instruments tremolo usually refers to variations in the amplitude rather than in the pitch of the tone produced) has a bridge that rotates relative to the body of the stringed instrument about a knife-edge hinge or rolling ball bearings to produce variations in the tension of the strings and thereby variations in the pitch of the tone.
Previously known vibrato assemblies have several disadvantages. Knife-edge hinges and rolling ball bearings have friction that can produce wear on the pivoting surfaces and cause hysteresis (i.e., prevent the strings from returning precisely to their basic pitch). The pivoting of knife-edge hinges and rolling ball bearings produces undesirable noise and rumbling sounds that nearby electro-acoustic pickups on electric stringed instruments detect and transmit to the amplifier. Knife-edge hinges and rolling ball bearings allow acoustic micro slip (i.e., sliding friction in the transmission of elastic strain waves) that prevents the efficient transfer of acoustic energy between the strings and the instrument body. This results in a loss of tonal quality (i.e., the number and relative intensity of the harmonics), frequency range, and sustain (i.e., an absence of energy loss that allows the string to vibrate freely). Also, because of the high line-contact or point-contact stresses present, even slight overloads can damage knife edges or ball-bearing races and thus cause increased friction, noise, and acoustic losses.
For the reasons previously discussed, it would be advantageous to reduce multiple acoustic paths that cause destructive interference and distortion and to selectively alter the acoustic attenuation. Additionally, it would be advantageous to have a vibrato assembly for stringed instruments that exhibits no wear or hysteresis, does not create extraneous noise, efficiently transfers acoustic energy from the strings to the instrument body, and withstands rugged use.